This invention relates to nuclear fuel assemblies of the type typically used in light water reactors, and more particularly, to features which improve the heat transfer between the fuel and coolant medium passing through the fuel assembly.
The importance of critical heat flux in the design of nuclear fuel assemblies and in the operation of commercial nuclear power plants is well known to practitioners in the field of nuclear power. Considerable design and testing efforts are directed toward optimizing the fuel assembly thermal hydraulic performance, while minimizing the effects of fuel assembly structure on the neutronics performance of the fuel. Typical fuel assemblies for commercial light water nuclear power reactors have a plurality of grids for defining square matrices of aligned and supported fuel rods which are spaced apart sufficiently to permit flow between the rods thereby to transfer heat to the fluid medium. The thermal-hydraulic optimization effort is directed toward maximizing heat transfer without approaching the critical heat flux, which condition results in a precipitous drop in the heat transfer coefficient between the rod and the fluid and a significant rise in the fuel clad temperature. Fuel assembly design changes that increase the critical heat flux provide the advantages of greater operating margin or increased core power rating.
Ideally, the velocity of the fluid is the same throughout the fuel assembly to guaranty that heat transfer from the fuel rods to the coolant fluid is maximized and local hot spots and the premature occurrence of boiling are prevented. This is important because premature occurrence of boiling reduces the possible level of energy production. Inherently, under axial flow conditions and without flow deflector structures, the flow velocity in the square matrices is less in the gaps between the in-line adjacent rods than in the wider gaps between the diagonally adjacent rods of the square matrix. Therefore, circumferential variations in the fuel rod heat transfer and temperature distributions inherently occur. In addition, variations also occur between different sides and regions of a given fuel assembly. Such variations are, at least to some degree, dependent on the location of the individual fuel assembly within the reactor core.
To effect some degree of improvement in approaching the above stated ideal, fuel designers have engineered a large variety of mixing and flow directing devices. The various mixing devices have helped by increasing the critical heat flux and thereby retarding the first onset of the critical heat flux anywhere on the assembly when compared to the assembly without the mixing device. A limiting factor, however, is that fuel assembly mixer designs must not increase the pressure drop across the grids beyond that which is tolerable.
For over two decades, it has been recognized that the provision of flow deflector structure on the fuel assembly grids can promote fluid mixing and thereby increase the critical heat flux. U.S. Pat. No. 3,379,619 entitled "Fuel Assembly for Nuclear Reactors" typifies the early flow mixing tabs carried by the grids. Many variations of this tab flow mixer, possibly numbering in the hundreds, have been used or proposed by practitioners in this field. Virtually all such variations are designed to promote "inter-subchannel mixing". A subchannel is defined as the fluid path, or "channel", which is more or less enclosed by three or four fuel rods. Fluid in one subchannel can mix with the fluid in neighboring subchannels through the gaps between fuel rods. Inter-subchannel mixing is the term used to describe this mixing between subchannels. These types of flow deflectors are effective for maintaining sub-cooled conditions by mixing liquid at different temperatures.
German Pat. No. 1,244,981 and U.S. Pat. No. 3,847,736 "Flow Twister For A Nuclear Reactor", exemplify the less common approach of using flow deflection means that have the effect of fluid mixing within the subchannel, with relatively low inter-subchannel mixing.
U.S. Pat. No. 3,589,438 "Spacer Grid For Heat Exchanger Elements With Eddy Diffusion Promotion Means" teaches hexagonal fuel rod cells 18 containing fuel rods 10. Irregular edges 28 and 29 of strips 17 defining the cells 18 act as flow deflectors.
U.S. patent application Ser. No. 843,525 filed March 24, 1986 by Parrette and Marshall and entitled "Nuclear Fuel Assembly Having Composite Mixing Grids" is assigned to the same assignee as the present application. The Parrette and Marshall application teaches an assembly in which a plurality of grids of a first type are spaced over the lower portion of a fuel assembly and at least one grid of a second type is located over these and has flow deflector structures associated with it for imparting a swirling motion within the flow subchannels to a greater extent than provided by the first type of grid. It does not, however, teach the present invention which includes a fuel assembly utilizing a box-type flow deflector grid to produce subchannel vortices.